1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device. More particularly, the present invention relates to an In-Plane Switching (IPS) mode LCD device.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices have been actively studied and researched because of their advantageous characteristics such as high contrast ratio, gray level, high picture quality and low power consumption. The LCD device is especially suitable as an ultra-thin display device such as a wall-mountable television. The LCD device has also attracted great attention as a display device that can be substituted for CRTs because the LCD device has a thin profile, light weight and low power consumption. As a result, the LCD device may be fabricated as a small panel and widely used for a mobile phone display.
Based upon the properties of liquid crystal and pattern structures there are various modes for LCD devices. Specifically, the LCD device may be categorized as a Twisted Nematic (TN) mode controlling liquid crystal director by applying a voltage after arrangement of liquid crystal director twisted at 90°, a multi-domain mode having a wide viewing angle by dividing one pixel into several domains, an Optically Compensated Birefringence (OCB) mode compensating a phase change of light according to a progressing direction of light by forming a compensation film on an outer surface of a substrate, an In-Plane Switching (IPS) mode forming an electric field parallel to two substrates by forming two electrodes on any one substrate, and a Vertical Alignment (VA) mode arranging a longitudinal (major) axis of liquid crystal molecule vertical to a plane of an alignment layer by using negative type liquid crystal and vertical alignment layer.
Among the various types of LCD devices, the IPS mode LCD device generally includes a color filter substrate and a thin film transistor array substrate facing each other, and a liquid crystal layer formed between the two substrates. The color filter substrate includes a black matrix layer for preventing light leakage, and an R/G/B color filter layer for realizing various colors on the black matrix layer. Also, the thin film transistor array substrate includes gate and data lines that cross to define a pixel region, a switching device formed at a crossing point of the gate and data lines, and common and pixel electrodes formed in an alternating pattern to generate an electric field parallel to the two substrates.
Hereinafter, a related art IPS mode LCD device will be described with reference to the accompanying drawings. FIG. 1 illustrates a plan view of a related art IPS mode LCD device of having a 2-domain structure. FIG. 2 illustrates voltage distributions of a related art LCD device along I–I′ of FIG. 1. FIG. 3A and FIG. 3B are plan views of an IPS mode LCD device when a voltage is turned on/off.
A thin film transistor array substrate of a related art IPS mode LCD device will be generally described as follows. As shown in FIG. 1, the thin film transistor array substrate includes a gate line 12, a data line 15, a thin film transistor TFT, a common line 25, a plurality of common electrodes 24, a plurality of pixel electrodes 17, and a capacitor electrode 26. The gate line 12 is formed in one direction on the thin film transistor array substrate, and the data line 15 is formed perpendicular to the gate line 12 to define a pixel region, wherein the data line 15 is formed to have a bent structure. Also, the thin film transistor TFT is formed at a crossing portion of the gate and data lines 12 and 15. Then, the common line 25 is formed parallel to the gate line 12 within the pixel region, and the plurality of common electrodes 24, each having a bent structure, extend from the common line 25. Also, the plurality of pixel electrodes 17 are connected to thin film transistor TFT, wherein each pixel electrode 17 is formed to have a bent structure that is parallel to the common electrodes 24, and provided between the common electrodes 24. The capacitor electrode 26 extended from the pixel electrode 17 overlaps the common line 25.
In addition, a gate insulating layer (not shown) is formed on an entire surface of the thin film transistor array substrate including the gate line 12, and a passivation layer (not shown) is formed on the entire surface of the thin film transistor array substrate including the data line 15.
The common line 25 is integrally formed with the common electrodes 24, and a gate electrode of the thin film transistor TFT is integrally formed with the gate line 12. Also, the common line 25, the common electrodes 24, the gate line 12, and the gate electrode are formed of a low-resistance metal on the same layer. The common electrode provided on the edge of the pixel region overlaps with the data line to prevent light leakage generated in the portion of the data line.
The pixel electrodes 17, extended from the capacitor electrode 26, are formed of a transparent conductive metal material having great transmittance, for example, indium-tin-oxide (ITO), wherein each pixel electrode 17 alternates with the common electrode 24. Also, the pixel electrode 17 is in contact with a drain electrode of the thin film transistor TFT, whereby the pixel electrode 17 receives a voltage.
As shown in FIG. 1, the common electrode 24 may alternate with the pixel electrode 17 in one direction. However, the common electrode 24 and the pixel electrode 17 may be formed in a zigzag type, to align liquid crystal molecules in two directions. That is, one domain of the pixel region may be divided into two parts to widen a viewing angle that is referred to as an S-IPS (Super-IPS) structure, the 2-domain IPS structure.
Also, a storage capacitor is formed of the capacitor electrode 26 overlapped with the common line 25, and the gate insulating layer and the passivation layer interposed between the capacitor electrode 26 and the common line 25. The storage capacitor maintains the voltage charged in the liquid crystal layer during turning off the thin film transistor TFT, thereby preventing degradation of picture quality.
In the related art IPS mode LCD device, as shown in FIG. 2, if 5V is applied to the common electrode 24, and 0V is applied to the pixel electrode 17, an equipotential surface is formed in parallel to the electrodes at the portions right on the electrodes, and the equipotential surface is formed perpendicular to the electrodes at the portion between the two electrodes. Accordingly, an electric field is perpendicular to the equipotential surface, whereby a horizontal electric field is formed between the common electrode 24 and the pixel electrode 17, a vertical electric field is formed on the respective electrodes, and both the horizontal and vertical electric fields are formed in the edge of the electrode.
An alignment of liquid crystal molecules in the related art IPS mode LCD device is controlled with the electric field. For example, as shown in FIG. 3A, if a sufficient voltage is applied to liquid crystal molecules 32 initially aligned at the same direction as a transmission axis of one polarizing sheet, long axes of the liquid crystal molecules 32 are aligned in parallel to the electric field. In case the dielectric anisotropy of the liquid crystal is negative, short axes of the liquid crystal molecules are aligned in parallel to the electric field.
More specifically, first and second polarizing sheets are formed on outer surfaces of the thin film transistor array substrate and the color filter substrate bonded to each other, wherein the transmission axes of the first and second polarizing sheets are perpendicular to each other. Also, an alignment layer formed on the lower substrate is rubbed parallel to the transmission axis of one polarizing sheet, whereby it is displayed on a normally black mode.
When voltage is not provided to the device, as shown in FIG. 3A, the liquid crystal molecules 32 are aligned to display the black state. Meanwhile, as shown in FIG. 3B, when voltage is provided to the device, the liquid crystal molecules 32 are aligned in parallel to the electric field, thereby displaying a white state. The common electrode 24 and the pixel electrode 17 are formed to have the bent structure, whereby the liquid crystal molecules 32 are aligned in two directions, thereby improving the viewing angle. However, because the liquid crystal molecules 32 are aligned in the different directions in the bent portion ‘A’ of the electrode, a disclination line may generate light leakage which degrades the picture quality.